1. Field of the Invention
This invention is directed to methods and apparatus for the catalytic oxidation treatment of waste gas streams that contain entrained solids and gaseous air pollutants such as volatile organic compounds and carbon monoxide, and particularly to methods and apparatus suitable for treating waste gas streams from wood burning processes.
2. Description of Related Art
Air pollutants, such as volatile organic compounds (VOC), carbon monoxide (CO) and oxides of nitrogen (NOx), are often controlled industrially by an incineration system that uses either a thermal or a catalytic process. Control of VOC and CO emissions is achieved by initiating oxidation reactions in these systems that convert the pollutants to harmless water and CO.sub.2. Control of NOx is often achieved by a selective reduction reaction which reacts ammonia with NOx to form N.sub.2 and water.
The abatement system is typically installed downstream of an industrial process to remove the pollutant constituents in the flue gas before the gas is emitted to the atmosphere. Thermal processes rely on homogeneous gas phase reactions for the destruction of these compounds, and normally operate at about 1500 to 1800.degree. F. (800-1000.degree. C.) with a residence time of about 1 second. On the other hand, the destruction reactions for catalytic processes occur at the catalyst surface rather than in the gas phase. Catalytic processes typically operate at about 600 to 1000.degree. F. (300-550.degree. C.) with a residence of time of about 0.1 second or less. Catalytic incineration systems are normally smaller in size, and consume less fuel than non-catalytic thermal systems.
Commercially, there are two general types of incineration designs, regenerative and recuperative, for either thermal or catalytic processes. Regenerative thermal oxidizer (RTO) or regenerative catalytic oxidizer (RCO) systems have very high thermal efficiency (&gt;90%). Recuperative thermal or catalytic oxidizers typically have a heat recovery of no greater than 70%. Selection of regenerative or recuperative type of oxidizers depend primarily on the exhaust concentrations and the exhaust flows, which also affect the operating and capital costs of the abatement system.
A typical regenerative thermal oxidation system is described in Houston, U.S. Pat. No. 3,870,474, incorporated herein by reference. In such a process, the VOC's and CO in a gas stream are incinerated at a relatively high temperature of about 1500.degree. F. Before entering the combustion zone, the gas stream passes through a first packed column of heat transfer packing material which heats the gas, and then exits through an identical second packed column which is heated by the gas from the combustion zone. Thus the hot gas exiting the combustion zone passes through a packed column, heating the packing material therein. Then the flow of the gas is reversed, and the incoming gas is heated as it passes through the packed column. By the use of such regenerative processes, the efficiency of thermal incineration has been greatly increased.
A drawback of such thermal oxidation systems is that they require heating the gas stream to the relatively high temperature of about 1500.degree. F. The 3,870,474 patent does indicate, at column 6, lines 3-7, that a suitable combustion catalyst may be placed in the warmest part of the regenerators to cause the contaminants in the air to be oxidized at a lower temperature.
For industrial processes that use wood materials or wood products as fuel, e.g. board mills, wood fired boilers, etc., the exhaust gas after combustion generally contains particulates as well as VOC and CO gaseous pollutants. Therefore, before the gas stream is directed to an incineration system to remove the VOC and CO pollutants, the gas is often treated to remove entrained particulate matter to prevent such particulates from interfering with the incineration process. Various methods are known for removing entrained solids from gas streams, as discussed in detail in the section on Gas-Solids Separations, Chemical Engineers' Handbook, Fifth Edition, 1973, pages 20-74 to 20-121. Such gas-solid separation systems, may be generally grouped into dry removal systems, in which equipment such as cyclones or electrostatic precipitators remove the dust from the air stream, and wet removal system, known as scrubbers, in which a liquid, usually water, is added or circulated to assist in the collection process (see Handbook, page 20-94).
In test operations involving RCO treatment of a flue gas stream from a wood burning process, the present inventors observed that in an operation in which particulates were removed prior to RCO by dry electrostatic precipitation (ESP), the catalyst quickly fouled and was rendered inactive. On the other hand, when the particulates were removed by a wet scrubber using water to assist in the collection process, the catalyst in an identical RCO system was observed to remain active indefinitely. There was therefore a need to determine why the catalyst fouled in one instance, and remained active in the other, and to develop a method for preventing the fouling of the catalysts in such catalytic oxidation systems.